The present invention relates to a power amplifier to provide protection using temperature sensing, in the event of faults occurring, for examples, when the output terminal is shortcircuited to a constant potential causing an output offset across the load or when an abnormal output offset condition, such as leakage cause from the deterioration of the coupling capacitor or the characteristic of the amplifier varies due to the influence of an environment occurs, and it could cause a DC offset current to flow through the load, such as a speaker, and the heat generated when the offset current flows through a voice coil in the speaker damages the speaker. Therefore the protection circuits will forestall the offset-induced adverse affects or damage to the power amplifier and the load. An example of such protection circuits is disclosed in the U.S. Pat. No. 2003/0160653 A1.
In a power amplifier system, it is desirable to avoid operation in fault modes, which could result in permanent damage of the system, especially power transistors. In a conventional design, the protection circuits detect and judge the occurrence of an offset using external low-pass filter and control unit.
FIG. 5 shows a prior art power amplifier system having an arrangement of protection to prevent a load, such as a speaker, from being damaged. The power amplifier system comprises a power amplifier 9 an electronic volume circuit 10, a coupling capacitor 11, a low-pass filter 12, a control unit 13 and a display unit 14.
The power amplifier 9 is provided with a non-inverting amplifying circuit 16 for applying non-inverting amplification to an audio signal supplied at the input terminal Q0 from the electronic volume circuit 10 through the coupling capacitor 11, an inverting amplifying circuit 17 for applying inverting amplification to the audio signal, a power amplifying circuit 18 for amplifying the audio signal from the non-inverting amplifying circuit 16, and another power amplifying circuit 19 for amplifying the audio signal from the inverting amplifying circuit 17.
The output contacts of the power amplifying circuits 18 and 19 are connected, respectively, to the output terminals Q1 and Q2 through switch elements 20 and 21. The output voltage S1 from the power amplifying circuits 18 and an output voltage S2 from the power amplifying circuits 19 are provided to a differential voltage detecting unit 22 for generating a differential voltage signal S12 for detecting an offset. The differential voltage detecting unit 22 has a differential amplifier for calculating a difference between the output voltages S1 and S2 and a comparator for comparing the voltage level of a difference signal (S1-S2) outputted from the differential amplifier with a certain threshold voltage VTHD. The comparator compares the threshold voltage VTHD with the differential signal (S1-S2), and outputs the differential voltage signal S12 in a duty ratio corresponding to the difference (S1-S2).
The low-pass filter 12 has a band as high as or lower than the audio band, and generates a detection signal Vdet of a DC voltage by smoothing the differential voltage signal S12 outputted from the outside output terminal Q4. The detection signal Vdet is supplied to the control unit 13.
The control unit 13 performs basic control. For example, control unit 13 adjusts a volume of the speaker 15 by receiving command information supplied from an operation panel or the like manipulated by the user, thereby controlling an operation of the electronic volume circuit 10 at a command from the users. The control unit 13 not only performs the basic control, but also pre-sets a threshold voltage Vok which is used for detecting the absence/presence of an offset. The threshold voltage Vok is determined in advance in consideration of a practically allowable offset voltage.
The control unit 13 compares the voltage level of the detection signal Vdet with the threshold voltage Vok, and judges the absence of an offset in the outputs from the power amplifying circuits 18 and 19 when the voltage level of the detection signal Vdet is higher than the threshold signal Vok. Conversely, control unit 13 detects the presence of an offset in the outputs from the power amplifying circuits 18 and 19 when the voltage level of the detection signal Vdet is equal to or lower than the threshold voltage Vok.
Upon judging the presence of an offset, the control unit 13 supplies a switching control signal to the switch elements 20 and 21 through the outside input terminal Q3, and brings the switch elements 20 and 21 into a non-conductive state (OFF state) to cut the connection between the power amplifying circuits 18 and 19 and the speaker 15. Upon judging the absence of an offset, the control unit 13 maintains the switch elements 20 and 21 in conductive state (ON state). The control unit 13 supplies signal to the switch display unit 14 to display a warning.
In this conventional design, the judgment of the occurrence of the offset and the control of the protection is accomplished by detecting the differential voltage, applying the detected differential voltage through the external LPF 12 and control unit 13 and feeding the switching control signal back to the power amplifier 9. This is operation is very slow and would stressed the power amplifier 9 and the speaker 15 before the protection starts. Thus, the performance is deteriorated.